A 54-year-old man presented with pollakiuria and dysuria. Ultrasonography, computed tomography (CT), and magnetic resonance imaging (MRI) showed a retroperitoneal tumor 10 cm in diameter. Pathological examination of a transrectal biopsy specimen obtained from the patient revealed the presence of an inflammatory myofibroblastic tumor (IMT). The patient underwent surgical resection of the tumor. Histopathologically, the tumor was composed of 3 parts : the largest part was compatible with an inflammatory pseudotumor: the second part, well-differentiated liposarcoma: and the third part, a myxoid liposarcoma. He has been followed up for 14 months ; but there was no evidence of local recurrence of the tumor. We believe that the inflammatory pseudotumor complicated in repairing the inflammation arising from the liposarcoma.